1. Field of the Invention
The present invention relates to a dot-matrix impact printer having a print head wherein a print wire or wires are driven by respective piezoelectric elements.
2. Discussion of the Prior Art
To obtain a high degree of printing quality in a dot-matrix impact printer, a gap between a recording medium and a print wire in its non-operated position (hereinafter referred to as "head gap") should be held at a contant value. If the head gap is excessively large, the printing pressure of the print wires may be insufficient, causing deterioration of the printing quality such as insufficiently colored imprints or partial or complete printing failure. If the head gap is relatively small, on the other hand, the printing pressure may be excessive, causing rapid wear of the print wire, and damage of the recording medium, an ink ribbon or other members.
In the light of the above inconveniences, the known impact printers are equipped with either a mechanism for moving or positioning the print head or a carriage supporting the print head, toward and away from the recording medium, or a mechanism for moving or positioning the recording medium toward or away from the print head. This arrangement allows the head gap to be kept at a constant value, irrespective of a change in the thickness of the recording medium.
Some dot-matrix impact printers use a print head which incorporates a piezoelectric element whose displacement upon application of a voltage thereto is utilized to drive the print wire, through a suitable amplifying mechanism for amplifying the amount of displacement of the piezoelectric element.
It is noted that a piezoelectric element has a linear negative coefficient of expansion due to a variation in its temperature, namely, the amount of expansion of a piezoelectric element in the direction of its displacement linearly proportionally increases with a decrease in its temperature. It is further noted that the amount of displacement of the piezoelectric element produced by a predetermined voltage applied thereto is constant, irrespective of the temperature of the element. Therefore, even with a constant operating stroke of a print wire between the non-operated (retracted) and operated (fully advanced) positions, these two positions are changed depending upon the ambient temperature of the print head. Accordingly, the head gap and consequently the printing pressure of the print wires are changed with the ambient temperature, even when the piezoelectric elements are energized with a constant voltage. This creates a variation in the density of the printed dot maxtrix pattern, a failure to produce imprints, or other troubles which lower the printing quality of the printer.
To solve the above drawbacks, the conventional dot-matrix print head employs a temperature compensation member made of a suitable metallic or other material which has a positive coefficient of expansion due to a temperature variation. This temperature compensation member is bonded to the piezoelectric element, or incorporated in the amplifying mechanism between the piezoelectric element and the print wire, so that the temperature characteristic of the piezoelectric element is compensated for by the temperature compensation member.
However, the above-indicated positioning mechanism for moving the print head, carriage or recording medium to maintain the constant head gap makes the printer to be complicated in construction, and large-sized and expensive.
It is also noted that the temperature of the piezoelectric elements is influenced not only by the ambient temperature, but also by the heat due to resistance losses of the piezoelectric element per se and the driver circuit, and the heat due to frictional wear of the print wire. In other words, the temperature of the piezoelectric element varies depending upon its duty cycle, and therefore tends to vary frequently. Thus, the adjustment of the positioning mechanism or amplifying mechanism to keep up with the frequently varying temperature is difficult to achieve, or the mere mechanical adjustment does not permit necessary compensation of the piezoelectric element for expansion or contraction due to the temperature variation.